1. Field of the Invention
This invention relates to methods for depositing silicon (Si) and germanium (Ge) in various manufacturing process, such as in semiconductor manufacturing. More particularly, this invention relates to methods for depositing epitaxial films such as epitaxial SixGe1-x films, in which x is in the range from zero to one.
2. Description of the Related Art
SixGe1-x films are used in a wide variety of semiconductor applications. An issue that often arises during the production of these materials is the lattice strain that may result from heteroepitaxial deposition. A “heteroepitaxial” deposited layer is an epitaxial or single crystal film that has a different composition than the single crystal substrate onto which it is deposited. A deposited epitaxial layer is said to be “strained” when it is constrained to have a lattice structure in at least two dimensions that is the same as that of the underlying single crystal substrate, but different from its inherent lattice constant. Lattice strain occurs because the atoms in the deposited film depart from the positions that they would normally occupy in the lattice structure of the free-standing, bulk material when the film deposits in such a way that its lattice structure matches that of the underlying single crystal substrate. For example, heteroepitaxial deposition of a Ge-containing material such a SiGe or Ge itself onto a single crystal Si substrate generally produces compressive lattice strain because the lattice constant of the deposited Ge-containing material is larger than that of the Si substrate. The degree of strain is related to the thickness of the deposited layer and the degree of lattice mismatch between the deposited material and the underlying substrate.
Strain is in general a desirable attribute for active device layers, since it tends to increase the mobility of electrical carriers and thus increase device speed. In order to produce strained layers on conventional silicon structures, however, it is often helpful to create a strain relaxed, intermediate heteroepitaxial layer to serve as a template for a further strained layer that is to remain strained and serve as an active layer with increased carrier mobility. These intermediate films are often provided by a relaxed SixGe1-x “buffer” layer over single crystal unstrained silicon (e.g., wafer surface), which can be engineered to provide the desired strain of an overlying layer (e.g., strained silicon layer).
Many microelectronic devices incorporate Ge-containing layers such as SiGe. To provide increased device performance, it is usually advantageous to have a relatively high germanium content in the SiGe layer. When deposited onto a single crystal Si substrate or layer, greater amounts of germanium generally increase the amount of strain. Generally, the higher the Ge content, the greater the lattice mismatch with underlying Si, up to pure Ge, which has a 4% greater lattice constant compared to silicon. As the thickness of the SiGe layer increases above a certain thickness, called the critical thickness, the SiGe layer relaxes automatically to its inherent lattice constant, which requires the formation of misfit dislocations at the film/substrate interface. The critical thickness depends upon temperature (the higher the temperature, the lower the critical thickness) and mismatch due to germanium content (the higher [Ge], the lower the critical thickness). For example, SiGe containing about 10% germanium has a critical thickness of about 300 Å at about 700° C. for an equilibrium (stable) strained film and about 2,000 Å for a metastable, strained film on Si<100>. If it is desirable to maintain the strain, the thickness is kept below the critical thickness and a cap layer is often applied to the strained heteroepitaxial layer to maintain the (metastable) strain of the SiGe layer during subsequent processing steps, e.g., to facilitate the formation of an emitter-base junction at the desired depth within the structure.
Although sometimes the relaxation is desired, when forming a buffer for subsequent strained deposition, the relaxation should be controlled to avoid some types of dislocations, such as vertically propagating or threading dislocations. Such dislocations lead to reduced carrier mobility, current leakage, reduced device performance and even device failure.
The quality of a deposited epitaxial layer generally depends on the cleanliness and crystal quality of the substrate onto which it is deposited. Since the substrate surface acts as a template for the deposited layer, any substrate surface contamination tends to degrade the quality of the deposited layer. Many epitaxial deposition processes employ a so-called “bake” step in which the substrate is heated to drive off surface contaminants such as oxygen and carbon immediately prior to epitaxial deposition.